In a wireless communication system, especially in a mobile communication system, fading occurs from times to times. Buildings, mountains, and foliage on the transmission path between a transmitter and a receiver can cause reflection, diffraction, and scattering on a propagating electromagnetic wave. The electromagnetic waves reflected from various large objects, travel along different paths of varying lengths. If there is an obstacle with sharp irregularities on the transmission path, the secondary waves resulting from the obstructing surface are present around the obstacle. Also if there are small objects, rough surfaces, and other irregularities on the transmission path, scattered waves are created. All these waves will interact with each other and result in multipath fading.
Under some environments such as in many metropolitan areas, there is no line-of-sight signal. The received signal is a multipath-fading signal from reflection, scattering, and diffraction. Statistically no any particular component of the multipath-fading signal is stable for a relatively long period of time and significant stronger than the rest components in a fairly large region. In order to provide service to these areas with good quality, all these major components of the multipath-fading signal have to be combined in some way so that on average the combined signal will be more stable and stronger than each component.
Before any attempting to combine the components of a multipath-fading signal, one has to identify all the significant components. This would require that all the significant components must be recognizable. That is, a transmitted symbol must be different from any of its neighbor symbols within the multipath-spanned range so that a delayed version of a transmitted symbol will be not mistaken as a different symbol.
A direct sequence spread spectrum system can naturally provide a way to distinguish the neighbor transmitted symbols and therefore all the significant components of a multipath-fading signal are recognizable. This is due to the fact that the delayed versions of the transmitted pseudo-noise (PN) signal have poor correlation with the original PN signal.
As IP originated messages are more and more popular, packet-switched communication system is more and more common. In a packet-switched communication system, a received package could come from total different source than the one before and the one after, and therefore generally there is no any relation between two adjacent packets. When transmission rate is very high, in order to reduce the capacity loss of communication system and obtain multipath information before the information loses its meaning, one could prefer to use matched-filter instead of correlator.
However, the regular matched filters do not work well. For a regular matched filter, the reference signal is fixed. In a direct sequence spreading spectrum communication system, the reference signal is changing all the time. Some modifications around the matched filter have to be made so that the matched filters are able to detect the various components of a multipath fading signal spanned over several symbol periods even though the reference signal is changing all the time.
The common method to combine several components of a multipath-fading signal consists of several steps. First, let the received signal pass a delay line with taps. The length of delay line should be long enough that the section of received signal captured by the delay line is equal or larger than range spanned by the multipath fading signal. Second, depending on the relative positions of the significant components, the several corresponding taps are selected. Third, the signal from each of selected taps is weighted by a different weight and is aligned properly by phase. And finally all the weighted signals are added together.
The common method of combinations has some drawbacks in a direct communication system.
First, when a multipath-fading signal spans for a large range and when the data rate is high, the above approach could consume a lot of hardware. In a direct sequence communication system, there are 64 chips in each symbol period and a multipath signal spans about 4 symbol periods. If one takes 4 samples in each chip, the delay line will consist of 4×64×4=1024 memory elements. Further suppose there are at most two significant components, in order to be able to select the two corresponding taps, two selective devices are needed with each one is connected to the 1024 memory elements. The selective devices consume a lot of hardware.
Second, a separate circuit could be needed to monitor each significant component. In order to keep good communication quality, one has to constantly monitor the information related to all the significant paths and adjust the weight, phase, and position associated with each significant component. But in a direct sequence spreading spectrum communication system, the signal from a selective tap does not directly provide the necessary information about that corresponding path. A despreading circuit has to be used to despread the signal of a selective tap and then the information about a corresponding path could be extracted.
In order to avoid huge selective devices and a plurality of despreading circuits, one perhaps prefers to despread each significant component by common despreading circuit first then combine these components together.
Based on previous discussions, it would be desirable to provide a mechanism for a receiver of a spreading spectrum communication system to despread all significant multipath components by a bank of matched filters, extract the related information from these despreaded components, and combine all the despreaded significant components efficiently.